1. Field of the Invention
The invention relates to image processing method and apparatus having the function of encoding or decoding a color image at high efficiency.
2. Related Background Art
Hitherto, as methods of encoding a color image, methods whereby an image is divided into a plurality of blocks and subjected to the orthogonal transformation and, thereafter, coefficients are quantized and encoded have been proposed by the same applicant as the present invention in U.S. Pat. Nos. 4,887,151, 5,157,743 and 3,162,923.
In the above conventional methods, however, since the coefficients after completion of the orthogonal transformation are quantized, the high frequency component is lost, a ringing occurs in the edge portion, and the quality of the character portion of an original deteriorates.
In the case of encoding at a low bit rate, a block distortion occurs at the boundary between blocks and an outline of a character becomes unsightly.
On the other hand, techniques to separately encode a plurality of color character portions and the other portions in order to improve qualities of black characters which are most frequently used and characters of a plurality of colors are proposed by the same applicant as the present invention in U.S. Pat. Nos. 5,146,606 and 5,361,147.
According to the above techniques, after the character portion and the other portions are separated, a DCT encoding is executed on the other portions.
In the DCT encoding, however, an image signal is converted into Y, C.sub.b, and C.sub.r signals, the Y, C.sub.b, and C.sub.r signals are respectively DCT encoded, the transformation coefficients are quantized and transformed into one-dimensional series, and Huffman encoding is executed. Therefore, even in the case of a "white" signal, namely, in the case where all of the input signal values in the block are equal to 255, a data amount of a total of three bits per block of at least one bit for each of the (Y, C.sub.b, C.sub.r) signals is necessary. Since the DC component which is obtained as a result of the orthogonal transformation uses another encoding (DPCM), a data amount of at least one bit per block is further needed, so that the loss of encoding efficiency is large.
When a diagram portion of a predetermined color of the input image data is extracted and the extracted diagram portion is encoded separately from the other portions, so long as only the diagram portion is merely eliminated, it is insufficient in terms of the accuracy of the extracting means or the like and, for instance, there is a case where the high frequency portion of the peripheral portion of the color characters remains. Such a problem also occurs in an image process such as a color converting process or the like other than the encoding process.